My 2 eldest were still pooing in a nappy at 3.5yrs.. I used to get anxious that they would never go in the toilet.. In the end I told them they needed to poop everyday or they would get a sore tummy and that they needed to ask for a nappy.. when they asked for a nappy I would say lets try sitting on the toilet for a little while and if nothing happens we will pop a nappy on

Another thing I did was empty the poo into the toilet and get them to laugh at the splash it made.. then we would giggle about how funny it looked in the toilet ect.. Eventually they got there